Wheel-support rolling bearing unit and a method manufacturing the same

ABSTRACT

A cylindrical portion  10  is not quenched, and its hardness of a cylindrical portion  10  is set at Hv 200  to  300 . An average cross sectional area of the crystal particle in this portion is set to be within  0.3  mm 2 . Accordingly, when a caulking portion is formed by expanding the cylindrical portion 10 b  to an inner ring  3  to a hub 2 b  made of carbon steel, cracks or the like are not formed in the resultant caulking portion.

BACKGROUND OF THE INVENTION

The present invention relates to a wheel-support rolling bearing unit,which is used for rotatably supporting vehicle wheels on suspensiondevices, and also a method for manufacturing the same.

The vehicle wheels are supported on the suspension devices by thewheel-support rolling bearing unit. A wheel-support rolling bearing unitis known which structured not using nuts for firmly coupling the hub andthe inner ring for the purposes of cost reduction by reducing the numberof the number of components parts, and the size and weight reduction, asdisclosed in Japanese Patent Unexamined Publication No. Hei.11-129703.FIGS. 4 through 6 show the wheel-support rolling bearing unit 1disclosed in the publication.

The conventional wheel-support rolling bearing unit 1 includes a hub 2,an inner ring 3, an outer ring 4, and a plurality of rolling elements 5.A first flange 6 for supporting the wheel is formed at a portion of thehub 2 which is located closer to the outer end of its outer peripheralsurface (the word “outer side or outside” means the outer side of thevehicle body as viewed in the widthwise direction in a state that theunit is assembled into the motor vehicle, and the left side in thosefigures except FIGS. 3 and 7. The word “inner side or inside” meanslocations closer to the center of the vehicle body as viewed in thewidthwise direction, and the right side in those figures except FIGS. 3and 7.). A first inner raceway 7 is formed in the outer surface of amiddle portion of the hub 2, and a stepped part 8 whose outside diameteris small is formed at the inner end thereof. The inner ring 3 is fit tothe stepped part 8 and fastened by a caulking portion 9. The first innerraceway 7 is formed directly in the outer surface of the middle portionof the hub 2, and sometimes is formed in the outer surface of a separateinner ring fit to the middle portion of the hub. In the later case, aportion of the end of the hub 2, which is protruded to the inner side asviewed in the axial direction beyond the separate inner ring, serves asthe stepped part which receives the inner ring 3.

To this end, a cylindrical portion 10, which is used for forming thecaulking portion 9, is formed in the inner end of the hub 2. A thicknessof the cylindrical portion 10 becomes small toward its top end beforethe cylindrical portion 10 shown in FIG. 6 is expanded, for caulking,outward in the diameter directions. Accordingly, a tapered hole 11 isformed in the inner end face of the hub 2. The inside diameter of thetapered hole becomes small toward a recess.

To expand, for caulking, the top end of the cylindrical portion 10 inorder to fasten the inner ring 3 to the inner end of the hub 2, a forcepiston 12 is forcibly pressed against the top end of the cylindricalportion 10 in a state that the hub 2 is fixed so as not to shift in theaxial direction, as shown in FIG. 5. An outward curved portion 13,shaped like a truncated cone, which may be thrust into the cylindricalportion 10, is formed in the central portion of the end surface (leftend surface in FIG. 5) of the force piston 12. An inward curved portionor recess 14 is formed surrounding the outward curved portion 13. Across section of the recess 14 is configured to have a complex curvedsurface whose radius of curvature becomes small toward the outside sothat a cross section of the caulking portion 9, which is formed byplastically forming the top end of the cylindrical portion 10 by therecess 14, gradually reduces in size from the base end to the top end,and abruptly reduces at the top end.

By pressing the force piston 12 having the outward curved portion 13 andthe recess 14, which are shaped and have dimensions as mentioned above,against the top end of the cylindrical portion 10, the top end of thecylindrical portion 10 is forcibly expanded outward in the diameterdirections for caulking, to thereby form the caulking portion 9. Theinner ring 3 is firmly held between the caulking portion 9 and a stepsurface 23 of the stepped part 8 formed at the inner end of the hub 2,whereby the inner ring 3 is fastened to the hub 2.

A first outer raceway 15 and a second outer raceway 17 are formed in theinner surface of the outer ring 4. The first outer raceway 15 is formedin the outer peripheral surface of the middle portion of the hub 2,while being confronted with the first inner raceway 7. The second outerraceway 17 is formed while being confronted with a second inner raceway16 formed in the outer peripheral surface of the inner ring 3. Aplurality of rolling elements 5, while being rollably held within anelement holder 18, are located between the first inner raceway 7 and thefirst outer raceway 15. A plurality of rolling elements 5, while beingrollably held within an element holder 18, are located between thesecond inner raceway 16 and the element holder 18. In the illustratedinstance, the rolling elements 5 are balls; however, in the case of thewheel-support rolling bearing unit for motor vehicles which is heavy,those may be tapered rollers.

To assemble the wheel-support rolling bearing unit 1 into the motorvehicle, the outer ring 4 is fixed to a suspension device with a secondflange 19 formed on the outer peripheral surface of the outer ring 4,and the wheel is fixed to the first flange 6. As a result, the wheel isrotatably supported on the suspension device.

For a work to form the caulking portion 9 by plastically deforming(expanding for caulking) the cylindrical portion 10 in order to form thewheel-support rolling bearing unit 1 thus constructed and operated, itis preferable to use a swing press 20 as shown in FIG. 7. The swingpress 20 is made up of a force piston 12, a holding tool 21 and a holder22. In forming the caulking portion 9 by expanding for caulking thecylindrical portion 10, the force piston 12 is displaced in a swingmanner while pushing upward the hub 2 with the aid of the holder 22. Ina state that the center axes of the force piston 12 and the hub 2 areinclined at an angle θ with respect to each other, the force piston 12is turned about the center axis of the hub 2, while being in contactwith each other. In forming the caulking portion 9 by the swing press asjust-mentioned, a part of the circumference of the force piston 12 ispressed against the cylindrical portion 10, so that the caulkingexpanding work of the caulking portion 9 continuously progresses in partin the circumferential direction. For this reason, in forming thecaulking portion 9 by ordinary forging process, a load acting on thecylindrical portion 10 may be reduced during the forging process. Theholding tool 21 prevents the inner ring 3 and the hub 2 from moving inthe radial direction during the caulking expanding work of the caulkingportion 9 by the force piston 12.

A technique that in the above structure, a portion of the outerperipheral surface of the hub 2, cross hatched in FIG. 4, is quenched tobe hardened to improve the durability thereof, is also disclosed in theJapanese Patent Unexamined Publication No. Hei. 11-129703. Specifically,the first inner raceway 7 portion, the base end portion of the firstflange 6, and the half of the base end portion of the stepped part 8 arequenched to increase hardness of those portions to about Hv550 to 900.On the other hand, the hardness of the cylindrical portion 10 which willform the caulking portion 9 is decreased to about Hv200 to 300 so thatthe cylindrical portion is easy to be plastically deformed.

Of those cross hatched portions to be quenched, the first inner raceway7 portion receives a great surface pressure upon contact of it with therolling surfaces of the rolling elements 5. Accordingly, it is hardenedorder to secure a rolling fatigue lifetime. The base end portion of thefirst flange 6 is hardened in order to prevent the base end portion frombeing deformed independently of a moment load receiving from the firstflange 6 to which the wheel is fastened. The half of the base endportion of the stepped part 8 is hardened in order to prevent the outerperipheral surface of the stepped part 8 from being deformedindependently of a fitting pressure of the inner ring 3 and a radialload that the inner ring 3 receives from the plurality of rollingelements 5, or to prevent the fretting from occurring on the outerperipheral surface of the stepped part 8 where it receives the innerring 3 fit thereto. The step surface 23 portion of the stepped part 8 ishardened in order to prevent the step surface 23 from being deformedindependently of an axial directional load acting on the inner ring 3 bycaulking work to be described later, to prevent the fretting fromoccurring on the step surface 23 as a contact surface where it comes incontact with the outer end surface of the inner ring 3, and to prevent acorner R as a continuous portion where the outer peripheral surface ofthe stepped part 8 continues to the step surface 23 from being deformedas the result of the stress concentrated thereto.

In a Japanese Patent Unexamined Publication No. Hei.10-95203, as shownin FIG. 8, the outside diameter of a portion of the inner end of the hub2 a, which is protruded beyond the fitting portion of the inner ring 3,is somewhat smaller than the outside diameter of the fitting portion.Specifically, a stepped part 25 having a height H of about 0.02 to 1 mmis formed on the outer peripheral surface of the base end of thecylindrical portion 10 a formed on the inner end of the hub 2 a at alocation closer to the second inner raceway 16 than a slanted surface24, which is formed at the inner end opening of the inner ring 3. Theslanted surface 24 is held down by expanding radially outwardly thediameter-reduced portion of the cylindrical portion 10 a for caulking.When the cylindrical portion 10 a is radially outwardly expanded forcaulking, the bending of it starts at the stepped part 25. Withprovision of the expanding work, excessive force hardly applies to thecylindrical portion 10 a during the caulking expanding work. As aresult, the expanded portion for caulking is little damaged, forexample, cracked.

The above-mentioned structure will possibly realize a small andlight-weight wheel-support rolling bearing unit 1 at low cost. However,to secure the sufficient durability, reliability and cost reduction ofthe resultant product, it is necessary to increase the production yieldby making the caulking portion 9 for fixing the inner ring 3 to the hub2 (2 a) free from damage, e.g., cracking.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide awheel-support rolling bearing unit which can prevent the caulkingportion from being damaged, for example, cracked, and realize sufficientcost reduction by the resultant increased production yield, and also toprovide a method for manufacturing the same.

The above-mentioned object can be achieved by a wheel-support rollingbearing unit, according to the present invention, comprising:

an outer ring having first and second outer raceways formed on its innerperipheral surface;

a hub made of carbon steel, the hub having a flange formed on the outerperipheral surface of one end thereof, the hub having a first innerraceway which is disposed on its middle portion in an axial directionthereof and is confronted with the first outer raceway of the outerring, the hub having a cylindrical portion which is disposed at theother end of the hub and has first and second portions, wherein anaverage cross sectional area of crystal particle in the second portionis less than 0.030 mm²;

an inner ring provided at the other end of the hub, the inner ringhaving a second inner raceway which is formed on its outer peripheralsurface and is confronted with the second outer raceway of the outerring;

a plurality of first rolling elements located between the first innerraceway and the first outer raceway; and

a plurality of second rolling elements located between the second innerraceway and the second outer raceway;

wherein a caulking portion, which is formed by plastically deforming thesecond portion of the cylindrical portion, is protruded beyond at leastthe inner ring fit to the hub, whereby the inner ring fit to the hub isfirmly coupled to the hub.

In addition, the above-mentioned object can also be achieved by a methodmanufacturing a wheel-support rolling bearing unit, according to thepresent invention, comprising:

an outer ring having first and second outer raceways formed on its innerperipheral surface;

a hub made of carbon steel, the hub having a flange formed on the outerperipheral surface of one end thereof, the hub having a first innerraceway which is disposed on its middle portion in an axial directionthereof and is confronted with the first outer raceway of the outerring, the hub having a cylindrical portion which is disposed at theother end of the hub;

an inner ring provided at the other end of the hub, the inner ringhaving a second inner raceway which is formed on its outer peripheralsurface and is confronted with the second outer raceway of the outerring;

a plurality of first rolling elements located between the first innerraceway and the first outer raceway; and

a plurality of second rolling elements located between the second innerraceway and the second outer raceway;

the method comprising:

preparing a blank hub which has the cylindrical portion including firstand second portions, wherein an average cross sectional area of crystalparticle in the second portion is less than 0.030 mm²; and

forming a caulking portion by plastically deforming the second portionof the cylindrical portion, thereby coupling the inner ring with the hubfirmly.

In the above-mentioned wheel-support rolling bearing unit or method, thehub may comprises:

a main body integrally formed with the flange portion; and

a separate inner ring which has the first inner raceway and is fit tothe main body.

In addition, in the above-mentioned invention, it is preferable that anaverage cross sectional area of the crystal particle in the secondportion is less than 0.020 mm².

Further, in the above-mentioned invention, it is more preferable that anaverage cross sectional area of the crystal particle in the secondportion is less than 0.0156 mm².

Furthermore, in the above-mentioned invention, it is advantageous thatan average cross sectional area of the crystal particle in the secondportion is less than 0.012 mm².

Further, the above-mentioned object can be achieved by a wheel-supportrolling bearing unit, according to the present invention, comprising: ahub having a first flange formed on the outer peripheral surface of oneend thereof, and a first inner raceway integrally or separately formedin the outer peripheral surface of a middle portion thereof; an innerring, provided at the other end of the hub, having a second innerraceway formed on the outer peripheral surface; an outer ring havingfirst and second outer raceways formed in the inner peripheral surface,the first outer raceway being confronted with the first inner racewayand the second outer raceway being confronted with the second innerraceway; and a plurality of rolling elements being located between thefirst inner raceway and the first outer raceway and a plurality ofrolling elements being located between the second inner raceway and thesecond outer raceway; wherein by a caulking portion, which is formed byradially expanding for caulking a cylindrical portion formed at aportion of the other end of the hub, which is protruded beyond at leastthe inner ring fit to the hub, the inner ring fit to the hub is firmlycoupled to the hub.

In the wheel-support rolling bearing unit thus constructed, the hub ismade of carbon steel. An average cross sectional area of crystalparticle in a portion of the cylindrical portion at which thecylindrical portion is plastically deformed (during the caulking portionforming work), is less than 0.030 mm². It is preferably less than 0.020mm², more preferably less than 0.0156 mm², and further preferably lessthan 0.012 mm².

For the hub, carbon steel is used which contains 0.45 to 1.10 wt. %carbon when the first inner raceway is directly formed in the outersurface of the middle portion of the hub. For the hub, carbon steel isused which contains 0.20 to 1.10 wt. % carbon when the first innerraceway is formed in the outer peripheral surface of the inner ring,which is separate from the hub. Examples of such a carbon steel are S53Cand S35C.

In the wheel-support rolling bearing unit thus constructed, the averagecross sectional area of the crystal particle of the carbon steel, whichmakes the cylindrical portion, which will become the caulking portion,is selected to be within 0.030 mm². During the work to form the caulkingportion by radially outwardly expanding the cylindrical portion, it iseffectively prevented that the caulking portion is damaged or cracked.As a result, in the wheel-support rolling bearing unit of the invention,the production cost can be sufficiently reduced while securing therequired durability and reliability.

Where the average cross sectional area of the crystal particle of thecarbon steel, is within 0.030 mm², defects which is problematic inpractical use will not be formed. When it is approximate to 0.030 mm²,sometimes creases, which are negligible in practical use, are formed inthe resultant wheel-support rolling bearing unit. When it is approximateto 0.020 mm², creases area also formed; however, the creases are muchsmaller and formation of them is more infrequent. When it is less than0.0156 mm², creases are little formed. When it is less than 0.0120 mm²,no or little crease is formed on the caulking portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing a part of a wheel-supportrolling bearing unit, which is an embodiment of the present invention,before the end of the hub is expanded for caulking;

FIG. 2 is a cross sectional view showing a part of the wheel-supportrolling bearing unit after its assembling is completed;

FIG. 3 is a diagram showing micrographs of two cases, one case in whichthe average cross sectional area of the crystal particle is small, andthe other case in which its cross sectional area is large;

FIG. 4 is a cross sectional view partially showing a conventionalwheel-support rolling bearing unit;

FIG. 5 is an enlarged, cross sectional view showing a state that theinner end of the hub is expanded for caulking in order to fix the innerring in a first structure when it is manufactured;

FIG. 6 is an enlarged, cross sectional view showing a part of thewheel-support rolling bearing unit before the inner end of the hub isexpanded for caulking;

FIG. 7 is a longitudinal sectional view showing a key portion of thewheel-support rolling bearing unit and a state that the inner end of thehub is expanded for caulking by use of a swing press;

FIG. 8 is an enlarged, cross sectional view showing a part of awheel-support rolling bearing unit which is another embodiment of thepresent invention;

FIG. 9 shows a cross sectional view showing, in a sequence ofmanufacturing steps, how defect is caused in the caulking expanding workwhen a positional relationship between the stepped part and the portionto be quenched is improper;

FIG. 10 is a longitudinal sectional view showing a key portion of thewheel-support rolling bearing unit and a state that the inner end of thehub is expanded for caulking by use of a swing press; and

FIG. 11 is a cross sectional view showing a part of a wheel-supportrolling bearing unit which is a modification of the embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 through 3 show an embodiment of the present invention. One ofthe characteristic features of the invention resides in a structurewhich prevents the deformation and wear of the respective parts andprevents part of the hub from being damaged, for example, cracked. Otherstructure than the above structure and a method of manufacturing thesame are similar to those in the conventional technique shown in FIGS. 4through 8. Therefore, description to follow will be given placingemphasis on the structure essential to the invention and those portionsdifferent from the conventional technique. Like reference numerals areused for indicating like or equivalent portions in FIGS. 4 through 8,for simplicity.

There is a modification of the above-mentioned embodiment that, as shownin FIG. 11, the first inner raceway 7 is formed in the outer peripheralsurface of a separate inner ring 3 a, which is fit to the middle of ahub 2 c (as a hub body), while not directly and integrally formed in theouter peripheral surface of the middle portion of the hub 2 c. In thismodification, a portion of it, which is protruded inward in the axialdirection, at the end of the hub 2 c, beyond the inner ring of thisseparate inner ring, is used as a stepped part to which the inner ring 3is fit.

A hub 2 b (FIGS. 1 and 2), which forms a wheel-support rolling bearingunit 1 a of the embodiment, is made of carbon steel. A surface portion(cross hatched in FIG. 1) of the hub including a stepped part 8 to whichthe inner ring 3 is fit is quenched to be hardened (hardness of thisportion is increased to about Hv550 to 900). The quenched portionincludes not only the half of the base end of the stepped part 8 butalso a region ranging from the first inner raceway 7 portion to the baseend portion of the first flange 6. A stepped part 25 is formed on theouter peripheral surface of a cylindrical portion 10 b, which isprovided on the inner end of the hub 2 b, whereby the base end (the leftside in FIGS. 1 and 2) of the cylindrical portion is large in diameterand the top end (the right side in FIGS. 1 and 2) of the cylindricalportion 10 b is small in diameter. The stepped part 25 has a height H ofabout 0.02 to 0.12 mm. The stepped part 25 has a curved surface arcuatein cross section, and is smoothly continuous in shape to at least thesmall-diameter portion (the outer peripheral surface of a portion of thecylindrical portion 10 b closer to its top end).

In the case of the hub 2 b which forms the wheel-support rolling bearingunit 1 a of the invention, crystal particles of the carbon steel of thecylindrical portion 10 b portion (not quenched, and located on the rightside of a chain line α in the figure) for forming a caulking portion 9,which is used for fastening the inner ring 3 to the hub 2 b, are 0.012mm² or smaller (it is 0.012 mm² in the FIG. 3A case) in an average crosssectional area before the caulking portion 9 is formed, as shown in FIG.3A. The average cross sectional area of the crystal particles of thecarbon steel, which forms the cylindrical portion 10 b portion, is setto have a predetermined value by adjusting a time from an instant thatthe hub 2 b is formed by forging till it is cooled down to normaltemperature.

In the case of the hub 2 b forming the wheel-support rolling bearingunit 1 a of the instant embodiment, a portion of the cylindrical portion10 b, which extends from the stepped part 25 and beyond the same, is notquenched. The quenched portion, cross-hatched in FIG. 1, terminates aposition closer to the base end of the cylindrical portion 10 b than thestepped part 25 (close to the step surface 23 and the left side in FIGS.1 and 2). Accordingly, the carbon steel, which forms the stepped part25, and the middle portion and the portion closer to the top end of thecylindrical portion 10 b, is in a green or half green state, and muchsofter than the quenched portion, cross hatched (its hardness is aboutHv200 to 300).

In the wheel-support rolling bearing unit 1 a thus constructed, thesurface portion including the portion of the stepped part 8 locatedcloser to its base end is hardened by quenching process. Accordingly,this surface portion is prevented from being deformed or damaged (e.g.,worn). Further, in the embodiment, the stepped part 25 is provided onthe outer peripheral surface of the cylindrical portion 10 b, Therefore,when the cylindrical portion 10 b is radially and outwardly expanded forcaulking, damage, e.g., crack, is hardly formed in the expanded portionin the caulking work.

In this embodiment, the average cross sectional area of crystalparticles, which form the cylindrical portion 10 to be the caulkingportion 9, is selected to be within 0.012 mm². Therefore, formation ofdamage, e.g., crack, in the caulking portion 9 is effectively preventedwhen it is formed by radially outwardly expanding the cylindricalportion 10 b. An experiment on this was conducted by the inventor. Thiswill be described with reference to FIG. 3.

In the experiment, two types of samples (as being blank hubs) were usedeach consisting of 100 samples, totally 200 samples. In one type ofsamples, the crystal particles of the carbon steel forming thecylindrical portion 10 b were 0.012 mm² in average cross sectional area,and in the other type of samples, those particles are 0.050 mm² inaverage cross sectional area. The cylindrical portion 10 b was expandedfor caulking for each sample. Inspection was made as to if the resultantcaulking portions 9 of those samples have defects. No defect was foundon all of those samples each having the average cross sectional area of0.012 mm² (inspection passing rate=100%). 40% of the samples each havingthe average cross sectional area of 0.050 mm² were defective (passingrate=60%). It the experiment, it was confirmed that if the average crosssectional area of the crystal particles of the carbon steel of thecylindrical portion 10 b is set to be small, a chance of making thecaulking portion 9 defective is effectively eliminated.

The portion extending from the middle portion of the cylindrical portion10 b and beyond its top end, which is the portion extending from thestepped part 25 and beyond the same, is not quenched. Therefore, theportion extending from the stepped part 25 and beyond the same is moreeffectively prevented from being damaged, for example, cracked in thecaulking expanding work. In other words, in the caulking expanding work,the stepped part 25 portion is reliably plastic-deformed radiallyoutwardly, and will tightly contact with the inner peripheral surface ofthe inner ring 3. The operation and effects of this will be describedwith reference to FIG. 9.

It is coupled with the first example of the conventional first structureshown in FIGS. 4 through 6, and the conventional second structure shownin FIG. 8, and the stepped part 25 formed on the outer peripheralsurface of the cylindrical portion 10 a is quenched. In this case,damage, e.g., crack, is likely to occur on the stepped part 25 when thecylindrical portion 10 a is expanded outward in the diameter directionsfor caulking. Specifically, when the hardened layer, cross hatched inthe figure, reaches to the stepped part 25 as shown in FIG. 9A, thestepped part 25 portion is hard to be plastic deformed when thecylindrical portion 10 a is expanded for caulking as shown in FIG. 9B.As a result, a groove 26 is formed and left in the outer peripheralsurface of the base end of the caulking portion 9 at a portioncorresponding to the stepped part 25, as shown in FIG. 9C. Crack or thelike is easy to be formed and its formation starts from the groove 26.

Turning to the embodiment, the stepped part 25 is not quenched.Therefore, the stepped part 25 is easy to be plastically deformed, sothat the groove 26 as shown in FIG. 9C is not formed. Particularly, inthe embodiment, the height H of the stepped part 25 is small, 0.12 mm.Accordingly, there is no chance of forming groove 26 which will causethe crack. Thus, the invention produces not only the cost reduction bydecreasing the number of component parts but also the increase ofproduction yield, which is caused by the reduction of a defectivearticle rate in the manufacturing stage. As a result, the cost ofmanufacturing the wheel-support rolling bearing unit 1 a issatisfactorily reduced.

Hereinafter, an one example of a manufacturing method for awheel-support rolling bearing unit according to the present inventionwill be explained with reference to FIG. 10. FIG. 10 shows alongitudinal sectional view showing a key portion of the wheel-supportrolling bearing unit and a state that the inner end of the hub isexpanded for caulking by use of a swing press. In FIG. 10, detaileddescriptions, as to components or structures which are substantiallyequal to or functionally coincided with them shown in the aforementionedembodiment, are eliminated by utilizing the same reference numerals.

A more preferable work to form the caulking portion 9 will be describedwith reference to FIG. 10. A protruded support 29, circular in crosssection, is provided at the central portion of the upper surface of abottom plate 27 of the holder 22 a, which forms a swing press 20 a. Acylindrical guide 30 is formed along the circumference of the uppersurface of the protruded support 29. A receiving tool 31 is placedwithin a space enclosed with the cylindrical guide 30. The top end face(upper end face) of the receiving tool 31 is thrust against the recessend face of a recess 32, which is formed in the central portion of theouter end surface of a hub 2 b. A cylindrical guide 28 to which acircular hole formed in the wheel as a vehicle wheel is fit is formed onthe outer end surface of the hub 2 b. The cylindrical guide 30 isprovided within the cylindrical guide 28.

The inside diameter of the cylindrical guide 30 of the holder 22 a isequal to or somewhat larger than the outside diameter of the cylindricalguide 28 of the hub 2 b. The inside diameter of a peripheral wall 33 ofthe holder 22 a is equal to or somewhat larger than the outside diameterof a first flange 6, which is formed along the peripheral surface of theouter end of the hub 2 b. Accordingly, in a state that the hub 2 b isset in the holder 22 a in order to form the caulking portion 9 on andalong the inner end of the hub 2 b, the outer peripheral edge of thefirst flange 6 is located close to and confronted with the innerperipheral surface of the peripheral wall 33, and the outer peripheralsurface of the cylindrical guide 28 is located close to and confrontedwith the inner peripheral surface of the cylindrical guide 30. And inthis state, the top end face of the receiving tool 31 placed on theholder 22 a is thrust against the recess end face of the recess 32,which is formed in the central portion of the outer end surface of thehub 2 b.

The hub 2 b is set to within the holder 22 a, a holding tool 21 a isfixed to the opening of the holder 22 a, and the inner ring 3 and thehub 2 b to which the inner ring 3 is fit are prevented from moving inthe radial direction. In this state, the force piston 12 is displayed ina swing manner while pushing upward the holder 22 a ( it is made to makea swing motion, like a precession, at an angle θ). The center axis α isinclined at θ with respect to the center axis B of the hub 2 b.Accordingly, a part of the circumference of the force piston 12 isbrought into contact with a part of the cylindrical portion 10 as viewedin the circumferential direction, which is formed on the inner end(upper end in FIG. 10) of the hub 2 b. The cylindrical portion 10 isexpanded for caulking in the radial direction while continuously movingthis contact portion in the circumferential direction, whereby acaulking portion 9 as shown in FIG. 10 is formed.

When the caulking portion 9 is formed by expanding the cylindricalportion 10 in this way for caulking, a thrust load acting on the hub 2b, which is based on the mutual pressing of the force piston 12 and thecylindrical portion 10, is received by the holder 22 a with thereceiving tool 31 being interposed therebetween. It never happens that alarge thrust load acts on the first flange 6, since the first flange 6,which is formed on the outer peripheral surface of the outer end of thehub 2 b, is fit to the peripheral wall 33 in a state that it isdisplaceable in the axial direction. Accordingly, the outer surface ofthe first flange 6, which may serve as a mounting surface on which thewheel and the disc rotor are mounted after the caulking portion 9 isformed on the inner end of the hub 2 b by the swing caulking, will notbe inclined with respect to a phantom plane, which is orthogonal to thecenter axis of the hub 2 b.

For this reason, if a required mounting accuracy of the first flange 6to the outer surface of the first flange 6 and the disc rotor issecured, both side surfaces of the disc rotor will not be inclined withrespect to the phantom plane, which is orthogonal to the center axis ofthe hub 2 b. Therefore, the judder, which occurs when the vehiclerunning at high speed is braked, can be reduced to zero or considerablyreduced.

With regard to the judder, if the outer surface of the first flange 6 isinclined and both the side surfaces of the disc rotor mounted on thisouter surface is inclined with respect to the phantom plane, which isorthogonal to the center axis of the hub 2 b, a violent vibration,called judder, occurs when the vehicle running at high speed is braked.Such a vibration makes the driver uncomfortable, and accordingly, shouldbe avoided.

While there has been described in connection with the preferredembodiment of the invention, it will be obvious to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the invention, and it is aimed, therefore, to cover inthe appended claim all such changes and modifications as fall within thetrue spirit and scope of the invention.

As seen from the foregoing description, the present invention isconstructed and operated as described above, the invention provides awheel-support rolling bearing unit which is low in weight and cost, andexcellent in durability.

What is claimed is:
 1. A wheel-support rolling bearing unit comprising:an outer ring having first and second outer raceways formed on its innerperipheral surface; a hub made of carbon steel, said hub having a flangeformed on the outer peripheral surface of one end thereof, said hubhaving a first inner raceway which is disposed on its middle portion inan axial direction thereof and is confronted with said first outerraceway of said outer ring, said hub having a cylindrical portion whichis disposed at the other end of said hub and has first and secondportions, wherein an average cross sectional area of crystal particle insaid second portion is less than 0.030 mm²; an inner ring provided atthe other end of said hub, said inner ring having a second inner racewaywhich is formed on its outer peripheral surface and is confronted withsaid second outer raceway of said outer ring; a plurality of firstrolling elements located between said first inner raceway and said firstouter raceway; and a plurality of second rolling elements locatedbetween said second inner raceway and said second outer raceway; whereina caulking portion, which is formed by plastically deforming said secondportion of said cylindrical portion, is protruded beyond at least theinner ring fit to said hub, whereby said inner ring fit to said hub isfirmly coupled to said hub.
 2. The wheel-support rolling bearing unitaccording to claim 1, wherein said hub comprises: a main body integrallyformed with said flange portion; and a separate inner ring which hassaid first inner raceway and is fit to said main body.
 3. Thewheel-support rolling bearing unit according to claim 2, wherein anaverage cross sectional area of the crystal particles in said secondportion is less than 0.0156 mm².
 4. The wheel-support rolling bearingunit according to claim 2, wherein an average cross sectional area ofthe crystal particle in said second portion is less than 0.020 mm². 5.The wheel-support rolling bearing unit according to claim 1, wherein anaverage cross sectional area of the crystal particles in said secondportion is less than 0.0156 mm².
 6. The wheel-support rolling bearingunit according to claim 1, wherein an average cross sectional area ofthe crystal particle in said second portion is less than 0.020 mm². 7.The wheel-support rolling bearing unit according to claim 1, wherein anaverage cross sectional area of the crystal particle in said secondportion is less than 0.012 mm².
 8. The wheel-support rolling bearingunit according to claim 2, wherein an average cross sectional area ofthe crystal particle in said second portion is less than 0.012 mm².